Background: Hepatitis B Virus (HBV) infection is a worldwide biomedical problem and an improved understanding of the assembly and structure of the virus may help develop new antiviral therapies as well as provide basic information on the structure of complex macromolecules. The HBV core gene codes for precore protein (pre-C) which is either partially processed to form a secreted non-particulate protein called e-antigen (HBeAg) or fully processed to produce core antigen (HBcAg). HBcAg is a 183-residue protein that encapsidate around a RNA-reverse transcriptase complex (HBV polymerase). HBcAg has been expressed in E.coli were it assembles in the bacterial cytoplasm into icosahedral capsids, which contain bound host nucleic acid. Deletion of the polybasic C-terminal 34 residues (protamine domain) also produces assembly competent protein. The capsids from C-terminal truncated protein (Cp149: residues 1-149) do not contain nucleic acid and their structure determined by cryo-electron microscopy and image analysis and by X-ray crystallography. Native HBeAg is also C-terminally truncated at position 149 and in addition contains a 10 residue N-terminal extension derived from partial processing of pre-C. Although the function and structure of HBeAg are unclear it is an important serological marker. Results: The assembly of the nucleocapsid protein has been further studied: residues which influence assembly has been identified by site-specific mutagenesis. A series of mutations were made in the core domain (Cp149) on the basis of available structural information. Residues which appear to be making contact between the subunits of the capsid were selected. One such mutation substituted a glycine residue for an alanine residue resulting in protein subunits which were unable to associate into capsids. Unassembled capsids are being studied to contrast the change in their immunological, biochemical and structural properties upon assembly. A nine amino acid sequence which links the shell forming (core domain) from the protamine domain also plays a key role in assembly. This linker peptide was located in the capsid structure by high resolution electron microscopy. The structural and biochemical evidence suggests the linker facilitates assembly of the capsid structure in the presence of bound viral nucleic acid. The immunological properties of Hepatitis B capsid are important clinically for diagnosis and monitoring the course of infection. Antibodies against various regions or epitopes of the capsid are also very useful tools for probing the structure. We are using biophysical techniques to measure the binding of various monoclonal antibodies to both assembled capsids and capsid subunits. In parallel, the structure of selected antibodies bound to capsids is being determined (A. Steven). The structural mapping of the major epitopes will soon be completed and will also provide basic information on the binding of antibodies to viral capsid structures. Significance and future direction: Other targets of HBV for structural determinations include the HBV polymerase and HBeAg these proteins have been expressed in E.coli and are being actively pursued. Unlike Hepatitis A and B, there is no vaccine against Hepatitis C. In the United States, Hepatitis C infection is reaching epidemic proportions, causing chronic liver diseases. The nucleocapsid of the virus which packages the viral genome is a critical component in the assembly of active virus. The capsids have been produced in bacteria were they self-assemble into particles of similar size to those found in the virus. Structural studies are being performed on the capsids. Summary: The Hepatitis B Virus (HBV) is the major worldwide cause of cancer. Although a vaccine is available, chronic HBV is often acquired in childhood. The HBV nucleocapsid plays an important structural role and metabolic role in the life cycle of the virus. An understanding of the molecular structure of the HBV nucleocapsid would allow targeted drug discovery with the aim of preventing the assembly and formation of the virus.